A company called Micro-Trim, Inc. has a great solution. You tell them the make and model of the m'wave, and the dimensions of the space where you're putting it, and they send you a custom kit with top and side strips that fill the space perfectly.
You can choose from three finishes, and the strips have ventilation openings so heat doesn't build up inside the cabinet space. You probably won't need any tools more advanced than a screwdriver. I had to raise up my oven on some 1 x 2" furring strips, but the whole job took just minutes, and it looks great.
I installed a kit for a Dacor Micro back in 2011. The same trim has also worked perfectly with the Sharp and Viking micros that followed, because they all share the same basic microbox. The price for the kit from Micro-Trim was significantly less than a kit from Viking.
Micro-Trim also has trim kits for cooktops and other appliances. Toll-free number is 800-338-8746. HIGHLY RECOMMENDED.